2. The Field of the Invention
This invention relates generally to optical transmission systems and devices. More particularly, exemplary embodiments of the invention concern optoelectronic semiconductor devices such as vertical cavity surface emitting lasers and vertical lasing semiconductor optical amplifiers.
2. Related Technology
As the result of continuous advances in technology, particularly in the area of networking such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of data over a telephone company's trunk lines, the transmission of images or video over the Internet, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth capacity will only increase.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the SONET protocol have been developed for the transmissions of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal. Optical amplifiers, which boost the power of the optical signal propagating through the system, are also a basic building block for fiber communications systems.
Many technologies exist for building the various components within fiber communications systems. For example, optical amplifiers may be created by doping a length of fiber with rare-earth metals, such as erbium, in order to form an active gain medium. The doped fiber is pumped to create a population inversion in the active medium. Then, when an optical signal propagates through the doped fiber, it is amplified due to stimulated emission. Transmitters may be a laser source modulated externally by a lithium niobate modulator, or a directly modulated DFB laser or FP laser. Semiconductor technologies are another alternative for building components. For example, laser sources may be created by using semiconductor fabrication techniques to build an optical cavity and a semiconductor active region within the optical cavity. The active region acts as the gain medium for the cavity. Pumping the active region results in a lasing optical cavity. In a variation of this theme, semiconductor devices may also be used as amplifiers. In one approach, an electrical current pumps the semiconductor active region of the amplifier, resulting in an increased carrier population. The optical signal then experiences gain as it propagates through the active region due to stimulated emission.
These types of semiconductor devices often will function in both the optical and electrical domains. For example, the amplifier described above functions as an optical device since an optical signal injected into the device is amplified as it propagates through the device. However, it also functions as an electrical device since the basic gain mechanism is an electrical effect (e.g., a p-n junction or multiple quantum well) and the device may also be electrically pumped. As a result, finding materials which can meet all of the optical and electrical requirements is difficult and, more often than not, more than one materials system is used to create the required device. For example, it is common to create devices which combine GaAs-based Bragg mirrors with InP-based active regions. However, the use of multiple materials systems creates its own problems, not the least of which is how to effectively combine the different materials systems. For example, GaAs and InP are not lattice matched and devices which combine the two typically will have a lattice mismatched interface somewhere, which can create undesirable effects. In addition, the techniques used to fabricate these devices may also lead to undesirable effects, such as the creation of deep-level defects, particularly if the device requires unusual or non-standard fabrication processes.
Thus, there is a need for semiconductor devices which overcome the undesirable effects generated by combining different material systems and/or by the processes used to fabricate the devices.